In establishing weld connections between an overlay foil or sheet and a carrier layer by means of high frequency energy, problems are frequently encountered when materials with very different dielectric constants are involved and when in particular the carrier layer represents the top layer of a thick laminate itself made up of a plurality of layers. One of the problems is that it is not easily possible to concentrate the high frequency energy exactly at the depth of the laminate where the heating and thus the welding is to be achieved. If for example the laminate is a foamed body to which a textile web is adhered which is to be welded to an overlay sheet at specific points the high frequency energy must be concentrated as far as possible in the region of the overlay sheet to ensure that the weld connection does in fact arise only between the overlay sheet and the textile web. Such laminates, which are welded to overlay sheets, are frequently used in the automobile industry as door inner linings in which certain areas are covered with an overlay sheet in the form of a plastic layer while in other areas a textile web matching the seat covers of the corresponding vehicle is exposed. To obtain an attractive appearance a clean weld joint must be established between the overlay sheet and the textile web and must surround the exposed area of the textile web. In the bagmaking industry as well it is frequently desirable to weld laminates of very different materials to an overlay sheet and the aim is always a clean appearance of the weld joint made.
In the present context it should be pointed out that the term "welding" is also used in the relevant art when, with the aid of high frequency energy, plastic sheets are heated up to melting and are pressed into textile materials so that the joint made is not a weld joint in the true sense in which both materials are heated above the melting point but a joint in which only the one material melts, penetrates into the other material and adheres to the latter by form locking.